Amplifier system



April 29, 1941.

G. L. USSELMAN AMPLIFIER SYSTEM Filed Dec. 16,

IIIIIIIIIlIIIIIIIII/I'IIl lIIIlIIlIIIII/IIIIIIIIIIIIIIIIII/IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII [III/IIIIIIIIIIIIIIIIIIIIII \Mmms INVEIV TOR. GEORGE L. USSELMAN ATTORNEI Patented Apr. 29, 1%41 ARKELEFHER SYSTEM George L. Usselman, Port .l'efferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application December 16, 1939, Serial No. 309,526

7 Claims.

This invention relates to improvements in ultra high frequency amplifier circuits.

One of the objects of the present invention is to provide an improved grounded anode type of ultra high frequency amplifier system, wherein the input circuit to the electrodes of the amplifier electron discharge device is inductively coupled to a low loss, low power factor oscillatory circuit.

Another object of the invention is to provide a multi-grid electron discharge device grounded anode amplifier system for use at ultra high frequencies, and wherein there is employed a tank circuit which is easily adjustable for use over a range of frequencies.

A feature of the invention lies in the use of a grounded anode amplifier wherein the anode of the vacuum tube is by-passed to ground for radio frequency energy, while other electrodes of the vacuum tube are coupled to the high current, low voltage portion of a low power factor oscillatory circuit.

Another feature of the invention lies in the use of a grounded anode multi-grid amplifier, whose filament heater leads and leads to the control grid and screen grid are enclosed in a metallic tubular conductor which, in turn, is inductive ly coupled to the inner conductor of a concentric resonant line.

Other objects and features and their advantages will appear from a reading of the following description, which is accompanied by a drawing wherein Figs. 1 and 1d, considered together, illustrate, by way of example only, a grounded anode amplifier in accordance with the principles of the invention. Fig. 1a is an enlarged view of the circuit elements appearing to the left of line QQ of Fig. 1. This enlarged view includes the ends of the coupling loop, which loop is located between the vacuum tube amplifier and part of the concentric line. Fig. 1 illustrates the true physical appearance of a side elevation of the coupling loop, while Fig. 1a. shows the ends of the coupling loop turned 90 to more clearly illustrate the circuit connections between the loop ends and the associated electrical circuit apparatus.

Referring to the drawing in more detail, Figs. 1 and 10., considered together, show an ultra high frequency amplifier system comprising a screen grid vacuum, tube V which is inductively coupled to a concentric resonant line YZ. This resonant line is constituted by a pair of conductors Y and Z suitably coupled together at one end by an end plate E and grounded at M. The screen grid tube V contains, within an evacuated envelope, a filament F, a control grid G1, a screen grid G2, and an anode A. The anode A is grounded for radio frequency energy to the outer conductor Y of the concentric line resonator by means of a by-pass condenser C9 located near the grounded end of the resonator. It will be appreciated that there is substantially no electric field outside of the concentric line conductor Y, and hence the outer conductor of the resonator is substantially at radio frequency ground potential. Condenser C9, in practice, may be constituted by a fiat metallic plate which is mounted upon the outer conductor Y but insulated therefrom by means of a suitable dielectric spacer, such as. mica or Bakelite.

The resonant concentric line Y, Z comprises a pair of coaxial conductors suitably connected together at one end by an end plate E, as shown, and whose electrical length is substantially onequarter wavelength at the operating frequency. The inner conductor Z of the concentric resonant line has its length maintained substantially constant with variation in temperature by means of a metallic bellows B located at the free end of the inner conductor to which is linked a low temperature coefficient rod RD, such as Invar, located within the inner conductor Z. In order to adjust the length of the inner conductor to provide a suitable adjustment in the resonant frequency of the resonant line, the rod RD is threaded at the end which passes through the end plate E and engages at this point a threaded nut. Handle H is attached to rod RD for turning it to adjust the position of the rod, thereby increasing and decreasing the length of the inner conductor. Such a resonant line is known in the art, reference being madeto the article entitled Resonant lines for frequency control, published in the Proc. of the I. R. E. for August, 1935, by Clarence W. Hansel], pages 852 to 857, for a more complete description thereof.

The electrodes of the vacuum tube V are inductively coupled to the inner conductor Z of the concentric resonant line at its high currentlow voltage end by virtue of a tubular metallic conductor P of any suitable cross-section. Pipe P surrounds the leads which connect to the vacuum tube electrodes and extends into the interior of the resonant line through aperture X in the outer conductor Y. Contained within pipe P is a coaxial transmission line JK and a plurality of other conductors L1, L2 and L3. The outer conductor J of this coaxial line is directly connected at both of its ends to the pipe or tubular metallic conductor P by means of connections N, while the inner conductor K of this coaxial line is connected at one end to the control grid G1 and at the other end to the secondary winding of a suitable transformer T1, the primary winding of which is in turn connected to the high frequency input circuit IN. The conductor L1 is connected at one end to the screen grid G2, and at this same end by-passed to the pipe P by means of a condenser C1, the other end of conductor L1 being similarly by-passed to pipe P by condenser Ca and connected to the positive terminal of a source of direct current potential through a suitable resistor R. The conductors L2, L3 are connected at one end to the legs of the filament F and at this same end by-passed to the pipe P by means of condensers C2, C3, and at the other end of pipe P similarly by-passed to the pipe by condensers C7, C8. At this last end, conductors L2 and L3 are connected to a source of heating energy S through a low frequency power supply transformer T2. Pipe P is grounded to the resonant line Y, Z at the end which is remote from the vacuum tube electrodes by means of connection D. The screen grid G2 is bypassed to ground by means of condenser C4. The control grid G1 is supplied with suitable negative bias over a path extending through conductor K, the aforesaid secondary winding of transformer T1 and lead U, which is connected back to the filament through a suitable by-pass blocking condenser 05. Condensers C1 to C9,. inclusive, form paths of relatively low impedance to energy of the operating radio frequency. A suitable output circuit is shown inductively coupled to the inner conductor Z of the concentric resonant line, also at its high current, low voltage end. This output circuit comprises a loop whose center point is shown grounded by means of a flexible connection W which serves to balance bothsides of the output circuit to ground.

, Connection Wis adjustable in position over the central portion of the loop 0 in order to obtain the desired voltage balance in the output circuit. This loop extends into the interior of the concentric resonant line through a suitable aperture X, as shown.

In the operation of the amplifier system, the vacuum tube V may be so biased normally that no anode current flows therein, or, if desired, r

high frequency oscillating potentials in the input I circuit IN connected to the primary Winding of transformer T1 will apply excitation potentials to the control grid G1. During the positive portions of the cycles of the applied oscillating potentials, electrons will flow to the anode A and screen grid G2, thus producing pulses of current in these electrodes. The current in the anode will flow through by-pass condenser C9 over the exterior surface of the outer conductor Y and then through the aperture X in' the outer conductor Y into the interior'of the resonant line. The screen grid current, however, will flow over conductor L1 and be by-passed to the pipe P by means of condensers C1 and C6. The current flow in the vacuum tube will vary the impedance between the filament F and the screen grid G2 and anode A, thus also producing a difference in potential between the filament F and the anode A, the latter being grounded for radio frequency energy at C9. Because of the fact that the screen grid G2 and the filament F are both by-passed to the pipe P at one end by means of condensers C1, C2 and C3, and the pipe P is grounded at its other end, it will be apparent that there will also be a corresponding difference of potential between the end of the pipe P nearest the vacuum tube and the grounded end of the pipe. Looking at it another way, the grounded end of the pipe P has the same reference potential as the anode A for radio frequency energy, and hence any potential differences between the filament F and the anode A in the vacuum tube are also applied between the end of the pipe P nearest the vacuum tube to which the electrodes F and G2 are bypassed and the anode or ground. The oscillating potential built up on pipe P between its two ends will cause an oscillatingcurrent to flow on its outer surface and by virtue of its coupling to the inner conductor Z of the concentric resonant line (through interlinking magnetic flux) will cause an oscillating current to flow in the interior of the resonant line Y, Z. The resonant line Y, Z will, of course, be tuned to the correct operating frequency by means of the handle H. The oscillating current in the resonant line Y, Z will then produce a current in the output loop.

If We assume the other operating condition stated above where the control grid G1 is so biased that normally steady anode current flows, then the application of high frequency oscillating potentials to the input circuit will bias the vacuum tube V to cut-off during the negative portions of the cycles of oscillating potentials, thus producing pulses of anode current. In this last case it was assumed that the amplitude of the high frequency input energy was such as to reduce the anode current to zero. If, however, the amplitude of the high frequency input energy is not suijcient to reduce the anode current to zero, there will be produced in the anode circuit an undulating anode current. The operation of the amplifier system insofar as the coupling between the vacuum tube and concentric resonant line and its output circuit is concerned, is the same as that described above in connection with the first condition.

We can summarize, briefly, the operation of the system of the invention as follows: A positive excitation pulse on the control grid causes a pulse or an increase of the direct current in vacuum tube V. This causes the cathode, screen grid and control grid to go more positive and approach the anode potential. Since the grounded end of pipe P is negative (with respect to the anode potential) and at this instant the other or free end of pipe P is positive, a pulse of radio frequency current will flow over pipe P. When the control grid goes more negative with respect to the cathode due to a negative excitation pulse, the vacuum tube current is reduced and the control grid with the screen grid and the cathode go more negative with respect to the anode and ground. This causes a reverse pulse of radio frequency current to flow over pipe P, Since the pipe P is coupled to the inner conductor Z of concentric lines Y-Z, an oscillating current will be caused to flow between and on the adjacent surfaces of the two conductors. The alternating potentials of the control grid and cathode are, of course, of like phase, as previously stated.

In some instances the amplifier of the invention may have some degeneration due to electron coupling of the control grid G1 to other electrodes of the vacuum tube, such as the anode. The circuit may be considered to be degenerative not only because the screen grid prevents regeneration by screening the control grid from the capacity infiuence of the anode, but also because the resistance effect of the D. C. tube current supplies a load to hold down the control grid excitation potential, This type of degeneration should be advantageous in preventing parasitic oscillations.

In the construction of the amplifier system of the invention it is preferred that the vacuum tube V be placed as closely as possible to the resonant oscillatory circuit, and where a resonant concentric line is employed for the oscillatory circuit it is preferred that the vacuum tube be mounted on the outer conductor Y with its electrode terminals extending into the interior of the resonant line. It should be distinctly understood, of course, that the oscillatory circuit may be any suitable type of low power factor, low loss tank circuit, and is not limited to the use of a concentric resonant line.

The amplifier of the invention may be employed in any stage in a communication system, although it will find most application in transmitters due to the large power obtainable by the amplifier, The invention may be employed either as an intermediate amplifier or as a final power amplifier.

The term ground used in the specification and claims refers to any point or surface of zero or fixed alternating current potential, and is not limited to an actual earthed connection.

What is claimed is:

1. In an amplifier system, an oscillatory circuit having substantially uniformly distributed inductance and capacitance, and comprising a pair of coaxial conductors coupled together at one end, a connection from ground to the outer conductor of said pair, a multi-electrode electron discharge device having an anode, a gridand a cathode, a connection for by-passing said anode to ground for energy of the operating frequency, a source of radio frequency energy to be amplified, and circuit connections from said electron discharge device to said source for exciting said grid at higher radio frequency potentials than but in phase with the potential of said cathode, said last connections being coupled to the inner conductor of said oscillatory circuit at a location of high current and low voltage, and an output circuit also coupled to said oscillatory circuit.

2. An amplifier system in accordance with claim 1, characterized in this that said output circuit and said electron discharge device are both inductively coupled to said oscillatory circuit.

3. In an amplifier system, a low loss, low power factor oscillatory circuit, a vacuum tube having an anode, a grid and a cathode, a connection for by-passing said anode to ground for energy of the operating frequency, a connection from ground to a point on said oscillatory circuit, a source of energy to be amplified, and circuit connections from said source to the electrodes of said tube for exciting said grid at higher potentials than but in phase with the potential of said cathode, said connections being so arranged as to inductively couple said vacuum tube to said oscillatory circuit at a portion of low voltage and high current on said oscillatory circuit, and an output circuit coupled to said oscillatory circuit.

4. In an amplifier system, an oscillatory circuit comprising a pair of coaxial conductors connected together at one end, a connection from ground to the outer conductor of said pair, a vacuum tube having a grid, a cathode and an anode, a connection from said anode to said outer conductor for by-passing energy of the operating frequency, a source of radio frequency energy to be amplified, a source of heating current for said cathode, connections respectively extending from said grid and cathode to said two sources, a hollow metallic conductor surrounding said connections, said hollow conductor with its enclosed connections extending into the interior of said oscillatory circuit through an aperture in the outer conductor and being in coupling relation thereto, said cathode connections being coupled to said hollow conductor at their points of ingress and egress by capacitive paths of low impedance to energy of the operating frequency, a connection from ground to a point on said hollow conductor near said points of egress, said grid being excited at higher -radio frequency potentials than but in phase with the potential of said cathode, whereby a potential diiference is impressed on said hollow conductor which corresponds to the potential difference between said anode and cathode, and an output circuit coupled to said oscillatory circuit.

5. In an amplifier system, an oscillatory circuit comprising a pair of coaxial conductors connected together at one end, a connection from ground to the outer conductor of said pair, a vacuum tube having a grid, a cathode and an anode, a connection from said anode to said outer conductor for lay-passing energy of the operating frequency, a source of radio frequency energy to be amplified, a source of heating current for said cathode, connections respectively extending from said grid and cathode to said two sources, a hollow metallic conductor surrounding said connections, said hollow conductor with its enclosed connections extending into the interior of said oscillatory circuit through an aperture in the outer conductor and being in inductive coupling relation thereto, said aperture being located at a place of high current and low voltage on said oscillatory circuit, said cathode connections being coupled to said hollow conductor at their points of ingress and egress by capacitive paths of low impedance to energy of the operating frequency, a connection from ground to a point on said hollow conductor near said points of egress, said grid being excited at higher radio frequency potentials than but in phase with the potential of said cathode, whereby a potential difference is impressed on said hollow conductor which corresponds to the potential difference between said anode and cathode, and an output circuit coupled to said oscillatory circuit.

6. In an amplifier system, an oscillatory circuit having substantially uniformly distributed inductance and capacitance, and comprising a pair of coaxial conductors connected together at one end, means for changing the resonant frequency of said oscillatory circuit over a range of frequencies, a connection from ground to the outer conductor of said pair, a multi-electrode electron discharge device having an anode, a grid and. a cathode, a connection for by-passing said anode to ground for energy of the operating frequency, a source of radio frequency energy to be amplified, and circuit connections from said electron discharge device to said source for exciting said grid at higher radio frequency potentials than but in phase with the potential of said cathode, said last connections being coupled to the inner conductor of said oscillatory circuit at a location of high current and low voltage, and an output circuit also coupled to said oscillatory circuit.

7. In an amplifier system, a resonant concentric line electrically one-quarter wavelength long at the operating frequency, comprising an inner conductor and an outer conductor connected together at one end, means for changing the resonant frequency of said concentric line, a connection from said outer conductor to ground, a vacuum tube having an anode, a control grid, a screen grid and a filament, a connection from said anode to said outer conductor for by-passing energy of the operating frequency from said anode to ground, a source of radio frequency en ergy to be amplified, a source of heating ourrent for said filament, a source of polarizing potential for said screen grid, connections respectively extending from said grid, filament and screen grid to said sources, a hollow metallic conductor surrounding said connections, said hollow conductor and its enclosed connections extending into the interior of said resonant line through an aperture in the outer conductor, said filament and screen grid connections being respectively coupled to said hollow conductor at their points of ingress and egress by capacitive paths of low impedance to energy of the operating frequency, a connection from ground to a point on said hollow conductor near said points of egress, said grid being excited at higher radio frequency potentials than but in phase with the GEORGE L. USSELMAN. 

